Fatty Acid Elongation Is Independent of Acyl-Coenzyme A Synthetase Activities in Leek and Brassica napus1

Abstract

In both animal and plant acyl elongation systems, it has been proposed that fatty acids are first activated to acyl-coenzyme A (CoA) before their elongation, and that the ATP dependence of fatty acid elongation is evidence of acyl-CoA synthetase involvement. However, because CoA is not supplied in standard fatty acid elongation assays, it is not clear if CoA-dependent acyl-CoA synthetase activity can provide levels of acyl-CoAs necessary to support typical rates of fatty acid elongation. Therefore, we examined the role of acyl-CoA synthetase in providing the primer for acyl elongation in leek (Allium porrum L.) epidermal microsomes and Brassica napus L. cv Reston oil bodies. As presented here, fatty acid elongation was independent of CoA and proceeded at maximum rates with CoAfree preparations of malonyl-CoA. We also showed that stearic acid ( Very-long-chain fatty acids (Ͼ18 carbons) are found in the glucocerebrosides of plant plasma membranes (Cahoon and Lynch, 1991), in the storage lipids of many oil seed species (Harwood, 1980), and as precursors of plant epicuticular waxes (Post-Beittenmiller, 1996). These fatty acids are synthesized by successive 2-carbon additions donated from malonyl-CoA. In each cycle of elongation, an acylprimer condenses with malonyl-CoA; this step is followed by a reduction of the 3-ketoacyl-CoA, dehydration of the hydroxyacyl-CoA, and reduction of the enoyl-CoA to the saturated and fully reduced acyl-CoA. These reactions are analogous to those of de novo fatty acid biosynthesis (Ohlrogge and Browse, 1995). However, unlike the soluble, stroma-localized enzymes of fatty acid synthase, the enzymes of acyl elongation are membrane bound and generally thought to be associated with the ER and possibly the plasma membranes in vegetative cells (von WettsteinKnowles, 1993) or with the ER (Agrawal et al., 1984) and oil bodies of developing seeds (Imai et al., 1995). Furthermore, the intermediates of acyl elongation are esterified to CoA rather than to the acyl carrier protein cofactor of fatty acid synthase (Fehling and Mukherjee, 1991).Studies on acyl elongation in plants and animals indicate that, in general, the properties of acyl elongation in plants and animals are quite similar (for review, see Cinti et al., 1992;Cassagne et al., 1994). Both plant and animal elongases require malonyl-CoA and NAD(P)H and use a variety of primers with varied requirements for ATP. For example, relatively high rates of acyl elongation can be achieved without supplied primer, i.e. by ATP-dependent elongation of endogenous substrates. This implies that there is a sufficient endogenous primer pool, but that ATP is necessary to use it.The extent of the ATP requirement for elongation of exogenous acyl-CoAs apparently depends on the system studied. In Brassica napus seeds both microsomes and oil body fractions have acyl-CoA elongation activity. The ATP-independent activity is found almost exclusively in the microsomal fraction (Fuhrmann et al., 1994), whereas the ATP-dependent activity is found in both microsomes...

Evidence that oleoyl‐CoA and ATP‐dependent elongations coexist in rapeseed (Brassica napus L.)

“…The ATP‐dependent elongation activity was restored by addition of CoA. As Lessire and Cassagne [18] have shown the presence of an acyl‐CoA synthetase activity in leek epidermis microsomes and as it has been shown that acyl‐CoA synthetase is stimulated by CoA [18,22], it has been hypothesized that the ATP‐dependent elongation could be due to activation of endogenous fatty acids to acyl‐CoAs which are further elongated. The involvement of the acyl‐CoA synthetase in elongation activity has been recently ruled out [22–24] as it has been shown that fatty acid elongation in leek microsomes and B. napus oil bodies was CoA‐independent even though acyl‐CoAs could be synthesized from [1‐ 14 C]C 18 : 0 in the presence of ATP and acetyl‐CoA or malonyl‐CoA.…”

“…Phosphatidylcholine has also been proposed as a precursor for elongases, as it has also been shown to be involved in palmitate elongation [23] and VLCFA transfer to the plasma membrane [24]. Recent results supported this hypothesis, as it has been observed that label from oleic acid appeared as rapidly in the phosphatidylcholine fraction as in the acyl‐CoA fraction [22]. However, Hlousek‐Radojcic et al .…”

Biosynthesis of Lipids and Hydrocarbons in Algae

“…Fatty acid elongation process is very similar to that of the fatty acid synthesis although acyl-CoA and malonyl-CoA are used as a substrate. In the process, 1ATP and 2NADPH are required for C 2unit elongation of saturated carbon chain since CoA-activation is not essential as suggested by another study (Hlousek-radojcic et al, 1998). Fatty acid elongation reaction site was shown to be located in the endoplasmic reticulum (Kunst & Samuels, 2009).…”

Peroxisomal Heat Generation and Possible Relationship With Peroxisomal Disorders

“…Elongase activity, enhanced in X-ALD, could require a low level of energy. In X-ALD heat could have a role in VLCFA homeostasis and have a link with the key role of acyl-CoA metabolism and with the mutated ALDP, which could be a heat-sensitive protein or a temperature stabilizer [11][12][13][14][15].…”